1. Field of the Invention
The present invention relates generally to optical devices and systems, especially to telecommunications systems, optical amplifier systems, and/or wavelength division multiplexing systems. The present invention also relates to devices for combining multiple optical input signals into one or more combined signals.
2. Description of the Related Art
Optical combiner devices are generally known. Such devices may be used to receive multiple pump signals via respective input ports and to combine the pump signals into an output signal. The input signals may have different operational wavelengths. The combined signal may be used to energize an optical amplifier, for example.
It has been suggested to locate fiber gratings upstream from the input ports of the combiner device to control and/or stabilize the wavelengths of the respective optical sources. One problem with this approach, however, is that it can be difficult to match the wavelength characteristics of the fiber gratings to the acceptance bandpass characteristics of the input ports. The spectral misalignment can be caused by normal manufacturing variations, by temperature variations, and by other factors. Any misalignment between the spectral characteristics of the gratings and the input ports of the combiner device can result in a loss of optical efficiency.
The present invention overcomes the problems of the prior art by providing a broadband partial reflector downstream from the combiner device. The partial reflector reflects a portion of the combined light signal back through the combiner device and back into the optical sources. There are no fiber gratings or other spectral filters between the sources and the combiner device. Consequently, the sources are locked and/or stabilized according to the acceptance bandpass characteristics of the input ports of the combiner device. In other words, the combiner device itself provides the spectral filters for feedback-controlling the wavelength characteristics of the sources. Since no gratings or other filters need to be located between the sources and the combiner device, the problem of power loss due to spectral mismatch at the input ports can be avoided.
In a preferred embodiment of the invention, an optical waveguide, such as an optical fiber, is used to transmit the combined light signal away from the combiner device, and the partial reflector is located within the waveguide or between the waveguide and the combiner device. That is, the partial reflector is located optically downstream from the combiner device. The partial reflector, together with the filtering properties of the combiner device (especially the input ports), causes the wavelengths of the input sources to be locked as a function of the acceptance bandpass characteristics of the respective input ports. In a preferred embodiment of the invention, the laser sources are locked to the different wavelengths of the input ports to support the most efficient transmission of optical power through the combiner device. The present invention should not be limited, however, to the preferred embodiments shown and described in detail herein.
The present invention also relates to a method of efficiently combining input light signals into a combined light signal. The method operates without any fiber gratings or other spectral filtering devices between the sources and the combiner device. Instead of gratings in the input fibers, the invention provides wavelength feedback stabilization by reflecting a portion of the combined light signal back through the input ports of the combiner device. The method operates to self-align the operational wavelengths of the laser sources to the acceptance bandpass characteristics of the input ports.
In a preferred embodiment of the invention, a single broadband feedback element is located after (downstream from) the combiner device. Thus, the combiner device may be located between the pump lasers and the broadband feedback element. The passbands of the combiner device (through the respective input ports) determine the wavelengths of the feedback light provided to the lasers, such that the lasing wavelengths are at the minimum loss wavelengths associated with the combiner device. Thus, efficient wavelength locking can be obtained independent of operating temperatures, age of the system, etc.